Blending apparatus



Aug. 23, 1966 T. A. BURTON 3,268,215

BLENDING APPARATUS Filed July 51, 1964 xii? 3 Sheets-Sheet l INVENTOR. 7%07714: $1

Aug. 23, 1966 T. A. BURTON BLENDING APPARATUS 3 Sheets-Sheet 2 Filed July 31, 1964 FILE. 5.

INVENTOR. 7%07774: fizz/r2271 BY j p ,qrrvw-o'z'Ksf Aug. 23, 1966 T. A. BURTON 3,268,215-

BLENDING I APPARATUS Filed July 31, 1964 5 Sheets-Sheet 5 INVENTOR PIE 2' 7 %077145 flfizzrz arz United States Patent 3,258,215 BLENDING APPARATUS Thomas A. Burton, Minneapolis, Minn., assignor, by means assignments, to Acheson industries, inc, Port Huron, Mich, a corporation of Michigan Filed July 31, 1964, Ser. No. 386,639 2 Claims. (Cl. 259180) The present invention broadly relates to a blending apparatus, and more particularly to an improved blending apparatus for mixing bulk quantities of different solid granular or particu-lated materials by withdrawing bulk quantities of the materials through supply conduits and discharging the materials thus withdrawn into a collector manifold or receiver which automatically regulates the gravitational flow of the material through each of the plurality of conduits.

The blending apparatus comprising the present invention constitutes a further improvement over the blending apparatus described in Uni-ted States patent application Serial No. 200,858, now Patent No. 3,158,362, filed June 7, 1962, for Method and Apparatus for Blending Granular Materials which is assigned to the same assi-gnee as the present invention. Blending apparatuses of the type disclosed in the aforementioned pending United States patent application and, as herein disclosed, are intended to overcome the disadvantages present in mechanical-type blenders such as drum-type mixers, for example, which heretofore have been employed for forming substantially homogeneous blends or mixtures of two or more dissimilar materials such as various types of grains, grits and abrasives, different colored plastic pellets, e'tc. Mechanical-type blending apparatuses, such as drum-type mixers, are subject to the inherent disadvantages of having relatively small capacities and requiring relatively high horsepower requirements over extended operating periods for effecting a substantially uniform mixture of several dissimilar granulated constituents. During the course of such mechanical blending, the particles being mixed are subjected to abrasion, causing attrition of the materials being mixed, and frequently producing an excessive quantity of unuseable fines which further detracts from the efficiency and economy of the mixing operation.

Various hopper-type blending apparatuses have heretofore been used or proposed for use to overcome the inherent disadvantages of mechanical blending apparatuses. In such hopper-type blending apparatuses, the granular material is either agitated by high-pressure fluid or is withdrawn from selected portions of a combination mixing and storage tank, and the admixed material is thereafter returned to the tank. In blending apparatuses of the hopper-type in which material is withdrawn from diverse locations in the hopper, a continuing problem has heretofore been presented by the difficulty of controlling the gravitational or pressure-induced flow of the granular material through each of the plurality of supply pipes. conventionally, unequal flow rates frequently result in stratification of the material in the storage hopper, resulting in either a non-uniform mixture or in excessively prolonging the time required for forming a uniform blend of the material. In order to overcome this problem, various intricate valving systems have heretofore been employed which require relatively careful manipulation and control to achieve the proper flow rates to provide a satisfactory mixing action. The cost, complexity, and susceptibility of such valving systems to malfunction, in addition to the relatively stringent regulation and control required of these valving systems during a blending operation, have seriously detracted from a more widespread adoption of such hopper-type blending apparatuses.

Patented August 23, 1966 In accordance with the invention disclosed in the aforementioned pending patent application, a receiver manifold is provided to which the material from each of the supply conduits is discharged and wherein the out-let ends of the supply conduits are disposed so asv to be partially blocked by the material discharged therein, providing therewith automatic flow control and dispensing with the necessity of requiring intricate valving systems of the types heretofore required. While the hopper-type blending apparatus as disclosed in the aforementioned United States patent application constitutes a significant advancement in hopper-type blending apparatuses, difficulty is still encountered in certain blending operations when a relatively uniform mixture of the various granulated materials is desired without any recirculation of the material to the hopper or with only a minimum of recirculation.

It is accordingly a principal object of the present invention to provide an improved hopper-type blending apparatus in which a more uniform sampling of the materials in the hopper is attained, overcoming the necessity of repeatedly recycling the admixed material back into the hopper to achieve a uniform mixture.

Another object of the present invention is to provide an improved hopper-type blending apparatus, the storage hopper or receptacle of which incorporates a plurality of supply conduits each of which is provided with a plurality of apertures therein at preselected vertically spaced increments, providing therewith a substantially uniform sampling of the stratified charge of material in the hopper during the initiation of the blending operation as Well as during the progress of the blending operation as the level of material in the hopper progressively decreases.

Still another object of the present invention is to provide an improved hopper-type blending apparatus which, in combination with the novel withdrawal means incorporated in the hopper and the collector manifold disposed therebelow, provides for a completely automatic and self-regulated blending of the granulated materials, achieving an efficiency and economy heretofore unattainable in blending apparatuses of the various types heretofore known.

The foregoing and other objects and advantages of the present invention are achieved by providing a hopper or tank in which a plurality of conduits are positioned, each formed with a series of apertures at vertically spaced intervals therealong for uniformly sampling and withdrawing the material from within the interior of said tank and discharging the material thus withdrawn into a collector manifold in which the material is combined, forming a substantially uniform mixture. In accordance with a preferred embodiment of the present invention, the receiving receptacle is positioned such that the outlets of each of the conduits are disposed at substantially the same level and wherein the material is Withdrawn from the receptacle at a rate so as to maintain the outlets of the conduits at least partially blocked with the material in the receiver, providing a self-regulated control of the withdrawal of material through each of the conduits.

Further objects and advantages will become more apparent from a consideration of the following detailed description taken in conjunction with the drawings in which:

FIGURE 1 is a vertical sectional view of a hoppertype blending apparatus constructed in accordance with the preferred embodiments of the present invention and taken along the line 11 of FIGURE 2;

FIG. 2 is an enlarged plan view of the blending apparatus shown in FIGURE 1;

FIG. 3 is a transverse sectional view of the lower end portion of the blending apparatus shown in FIGURE 1 and taken along the line 33 thereof;

FIG. 4 is a fragmentary enlarged vertical sectional view of the lower portion of the hopper or receptacle and the receiver disposed therebelow, illustrating the dynamic disposition of the granulated material during the course of a blending operation;

FIG. 5 is an enlarged fragmentary side elevation view of the lower ends of the supply conduits and the shape of the material being discharged therefrom into the receiver;

FIG. 6 is a transverse sectional view of a supply conduit formed with an inlet port therein which may be selectively opened or closed by means of a slotted sleeve valve;

FIG. 7 is an elevation view of the hopper of the blending apparatus, schematically illustrating a typical fiow pattern of the stratified material charge into the inlet ports of the supply conduits incorporated therein; and

FIG. 8 is a side elevation view similar to that illustrated in FIGURE 7 illustrating a typical flow pattern of the material after approximately one-half thereof has been withdrawn from the hopper.

Referring now in detail to the drawings, and as may be best seen in FIGURES 1 and 2, a blending apparatus constructed in accordance with the preferred embodiments of the present invention consists of a storage tank or hopper 10 which is supported from brackets 11 attached to the upper sides thereof by a suitable frame (not shown) which maintains the tank in a substantially upright position. The exemplary tank as shown in the drawings is of a circular-cross-sectional configuration and includes a conical base portion 12, a substantially cylindrical intermediate portion 14, and a convex dome or top 16. While a tank of a circular cross-sectional configuration is usually preferred, alternative satisfactory configurations such as square, hexagonal, elliptical, rectangular, etc., can also be satisfactorily employed. The dome or top 16 of the hopper 10 is formed with a centrally located flanged port 18 which is provided with a removable closure member or cover 20 for gaining access into the interior of the hopper as well as for loading the granulated or particulated materials to be admixed. In addition, a pair of auxiliary flanged ports indicated at 22 are also provided on the top 16 which may be directly connected to suitable conduits for charging the materials to be mixed directly into the interior of the hopper.

A plurality of delivery pipes or supply conduits 24 are positioned within the interior of the hopper 10 and extend upwardly therethrough and are secured to the dome or top 16 of the hopper. The lower portions of the supply conduits 24 extend through the conical base portion 12 of the hopper and terminate with the discharge .ports thereof indicated at 26 disposed in communication with the interior of a collector manifold or receiver 28 connected to and disposed directly below the conical base portion 12. In the preferred embodiment of the blending apparatus as illustrated in the drawings, each of the discharge ports 26 of the supply conduits 24 are disposed in substantially the same horizontal plane for providing automatic regulated control of the discharge or flow of material out through the discharge ports 26 in a manner subsequently to be described.

The collector manifold or receiver 28, as best seen in FIGURE 1, consists of a substantially cylindrical upper portion 30 which is integrally connected to a conical base portion 32 which is formed with a discharge outlet 34 in the base thereof through which the material received in the collector manifold is discharged into a suitable container indicated at 36 in which the discharge material can be transported or through which the material can be conveyed to the next processing operation or packaging operation. It is also contemplated within the scope of the present invention that the material in the container 36 can be recirculated back into the interior of the hopper 10 if desired, although such recirculation is ordinarily not necessary to achieve a substantially uniform blend of the materials charged to the hopper. The

discharged outlet 34 is provided with a suitable gate valve assembly indicated at 33 including a slidably mounted plate or gate 4%) and a pivotally mounted lever 42 connected thereto for adjustably positioning the gate 40 relative to the discharge outlet 34. The collector manifold 28 is also provided with a flanged access port 47 mounted on the periphery of the cylindrical upper portion 36 thereof, which is provided with a suitable removably mounted closure member or cover 46 for gaining access to the interior of the receiver as may be required from time to time.

In the specific form of the invention as illustrated in the drawings, the supply conduits 24 are arranged in a circular pattern disposed concentric to the wall of the cylindrical intermediate portion 14 of the hopper 10 and are positioned at substantially equal arcuate increments. The lower end portions of the supply conduits 24, as best seen in FIGURE 3, are similarly positioned in substantially equal circumferential increments adjacent to and concentric with the wall of the cylindrical upper portion 30 of the collector manifold 28. Each of the supply conduits 24 extending upwardly into the interior of the hopper 10 is formed with one or a plurality of inlet ports indicated at 48 which are disposed at vertically spaced increments thereal-ong for uniformly sampling and withdrawing the granulated material from the interior of the hopper and for discharging the withdrawn material into the collector manifold therebelow. The specific number and the vertical location and spacing of the inlet ports in each supply conduit 24 as well as the specific arrangement of the supply conduits 24 Within the hopper will vary in accordance with the configuration, volume, and height of the hopper as well as the nature of the materials being blended. The specific location of the inlet ports 48 along the length of each of the supply conduits is selected so that geometrically similar sections of the material in the hopper are disposed in communication with an inlet port whereby the solid granulated or particular material is withdrawn substantially equally through each of the inlet ports providing for a uniform sampling of the initial stratified or layered charge regardless of the level of the material in the hopper. In accordance with this relationship, samples are withdrawn from the granulated material in the hopper from equal geometrically similar sections thereof and are discharged into the collector manifold in which they are combined, forming an accurate composite of the entire contents of the material in the hopper.

The foregoing relationship is based on the discovery that while a plurality of vertically spaced inlet ports are provided in each supply conduit, only the uppermost of these inlet ports disposed below the level of the material in the hopper is operative to withdraw material while the remaining inlet ports in that conduit spaced vertically below are rendered substantially inoperative and are blocked by the material fiowing downwardly through the conduit. The relationship is maintained, provided that the flow rate of material through the supply conduits is less than the unobstructed free-fall rate of the particles.

The term free-fall rate as herein employed refers to the unobstructed gravitational falling rate of the particles downwardly through the supply conduits. The rate of withdrawal of material from the tank through the supply conduits in accordance with the practice of the present invention is less than the free-fall rate whereby each of the conduits are substantially completely filled with material downwardly of the inlet port through which the material is withdrawn.

While the static and dynamic forces governing the controlled withdrawal of material from the tank are not completely understood at the present time, it is believed that the downward movement of the particles in a supply conduit at less than free-fall conditions wherein the conduit is completely filled with the particles exerts an outward force at the inlet ports disposed below the uppermost inlet port which substantially balances the static inward force of the particles adjacent to such lower inlet ports, preventing any appreciable inward flow of granular material into such lower inlet ports. As a result, a dynamic equilibrium condition is attained adjacent to such lowermost inlet ports positioned below an operative inlet port until such time that the level of the material in the hopper decreases, exposing the uppermost operative inlet port whereupon the next vertically spaced inlet port becomes operative after the material in that supply conduit decreases to a level corresponding to the vertical height of the next inlet port. Accordingly, as the level of the material progressively decreases in the hopper, each of the inlet ports disposed along a given supply conduit become successively operative to withdraw material from the interior of the hopper until such time that the level of the material drops below such inlet port.

It will be appreciated from the foregoing that the number and location of the inlet ports along the lengths of the supply conduits provide for a continuous withdrawal of material from geometrically similar sections of the granular material regardless of the volume or level of the material in the hopper. As the level of the material progressively decreases, the geometrically similar sections or volumes of the material sampled also decrease proportionately. While the specific relationship of the number of supply conduits, their positions in the interior of the hopper, and the number and location of inlet ports along the length thereof can be mathematically computed to provide the requisite uniformity in sampling of the contents of the material charge in the hopper, mathematical estimates combined with a trial-and-error testing have been found to enable the attainment of extremely accurate uniform mixtures of a plurality of dissimilar materials without the necessity of recirculating the admixed material.

In order to enhance the flexibility and versatility of the blending apparatus comprising the present invention, it is contemplated that each of the supply conduits can be provided with a plurality of inlet ports 48 at substantially equal vertically spaced intervals therealong and that selected ones of the inlet ports along each supply conduit are rendered inoperative by adjustably positioning a sleeve 50 as best seen in FIGURE 4 over the selected ones of the inlet ports in order to provide the requisite sampling relationship consistent with the number and characteristics of the particular mate-rials to be blended. As shown in FIGURE 4, the sleeve 50 is slidably mounted around the periphery of the supply conduit 24 and is retained in appropriate vertical adjusted relationship thereon by means of a suitable set screw 52. Alternatively, the sleeve 50 may comprise a tubular segment having a longitudinal portion of the wall thereof removed corresponding substantially to the arcuate width of the inlet port and enabling selected rotation of the sleeve so as either to open communication of the inlet port with the interior of the hopper or to completely close the port as may be desired. This latter arrangement is illustrated in FIGURE 6 wherein a slotted sleeve 54 is slidably disposed around a supply conduit 24 having an inlet port 48 in one wall thereof and which sleeve 54 is adjustably positionable from an open position as shown in solid lines in FIGURE 6 to a closed position as shown in phantom, effecting a complete mechanical blocking of the inlet port. Appropriate adjusted positions of the slotted sleeve 54 can be conveniently maintained by means of a set screw indicated at 56.

A typical flow pattern of the particulated material into the inlet ports of the supply conduits in accordance with the arrangement and relationships as hereinabove set forth is best illustrated in FIGURES 7 and 8. As shown in FIGURE 7, the tank or hopper is filled with three layers of a stratified charge generally indicated at A, B and C. The supply conduits which correspond to those shown in FIGURE 1 in accordance with the section taken along the line 11 of FIGURE 2, numbering nine in all, are designated as C1 through C9, respectively, and the inlet ports therealong commencing with the uppermost in each supply conduit and moving downwardly therealong are numbered respectively such as, for example, C11, C12, C1-3, etc. In the condition as shown in FIGURE 7, the upper layer of charge material A is withdrawn from the hopper through the inlet ports C1-1, C4-1, C5-1 and C9-l. The material B in the intermediate stratified layer is withdrawn through inlet ports C21, C31, C7-1 and C8-l. The material C in the lower portion of the hopper 10 is withdrawn from inlet port C61 and through a central outlet 58 in the base of the tank 10.

The material withdrawn from the various inlet ports is discharged out through the discharge ports 26 into the collector manifold therebelow in which it is consolidated and withdrawn therefrom into a suitable rewiver 36, as illustrated in FIGURE 1. The inlet ports, as will be noted in FIGURE 7, which are spaced below an operative inlet port in the same supply conduit such as, for example, the inlet ports 01-2, C1-3, C1-4 and C1-5 of the supply conduit C1, are rendered substantially inoperative by the downward flow of material through the conduit C1 at less than a free-fall rate as it is withdrawn through the operative inlet port Cit-1. As the level of the material progressively decreases during the course of the blending operation, the stratified materials A, B and C attain an intermediate level, as diagrammatically indicated in FIGURE 8. During the course of the downward movement of the material, inlet ports Cll1, C3-1, C44, C4-2, C51 and C91 have become exposed and are rendered inoperative, whereby the next inlet port in each of the supply conduits disposed below the exposed inlet port commences to withdraw the material remaining in the hopper. Accordingly, in the condition shown in FIGURE 8, the stratified layer of material A is withdrawn by inlet port Cl-Z and C8-1 as well as by inlet ports CZ-l and C9-2 which overlie the stratified charges A and B. The layer of material B is withdrawn through inlet ports C5-2 and C7-1 in addition to inlet ports C21 and C9-2 which straddle the interface between the layers A and B. The material C in the base of the tank is withdrawn through inlet ports C32, C4-3 and C6-1 in addition to the central outlet 58 in the base of the tank.

In the flow patterns as exemplified in FIGURES 7 and 8, material is being constantly withdrawn from geometrically similar sections of the charge in spite of a progressive decrease in the total quantity of material in the tank whereupon the composite material discharged into the collector manifold is an accurate sampling of the contents of the interior of the hopper. As the material level progressively decreases from the position as shown in FIGURE 8, the inlet ports disposed below those inlet ports currently active are rendered operative and finally the remaining material is withdrawn through the lowermost inlet ports designated Cl-S, C2-4, C32, C4-4, C5-4, C62, C7-2, C8-4 and C9-4. When the level of material drops below the level of the aforementioned lowermost inlet ports, the balance of material is withdrawn from the tank by the central outlet 58 in the base thereof.

It is contemplated in accordance with the present invention that the central outlet 58 in the base of the tank can be entirely omitted, resulting in a dead area such that the level of material withdrawn would stop when lowermost inlet ports are exposed. Alternatively, a suitable gate valve indicated at 60 in FIGURE 1, can be incorporated on the central outlet 58 similar to the gate valve assembly 38 at the base of the collector manifold 28, enabling a closure of the outlet port 58 during the course "of the tank 10, can be modified in configuration so as to minimize the dead space disposed below the lowermost inlet ports in the several supply conduits.

When a central outlet 58 is employed consistent with the blending cycle as described in connection with FIG- URES 7 and 8, the outlet opening thereof is of a crosssectional area such that the volume of granular material withdrawn therethrough during the course of a blending operation is equal to the volume of material disposed below the lowermost inlet ports in the supply conduits, during which time the material in the storage hopper or blending tank disposed above the lowermost inlet ports is withdrawn through the several inlet ports of the several supply conduits. Accordingly, the material located in the base of the tank is uniformly withdrawn in a direct proportion to the material being withdrawn through the various supply conduits providing therewith an accurate sampling and uniform mixing of the several materials.

In accordance with the preferred practice and construction of the blending apparatus comprising the present invention, the flow of material discharged from the discharge ports 26 of each of the supply conduits 24 is regulated by means of a controlled withdrawal of the composited material from the collector manifold 28. The withdrawal of material from the collector manifold 28 provides for a partial blocking of the discharge outlets 26 and the central outlet 58, providing thereby an automatic regulated flow of material and assuring that the material withdrawn passes downwardly through the supply conduits 24 at a velocity less than the free-fall rates of the materials. The discharge outlet 34 in the base of the collector manifold 28 is of a size to assure that when the gate 40 is completely withdrawn the flow of material out of the collector manifold will be less than the free-fall rate of withdrawal or delivery of material from all of the supply conduits 24 and the center outlet 58. This assures a dynamic partial blocking relationship of the discharge ports 26 in the base of the supply conduits 24 and of the central outlet 58 as is illustrated in FIGURES 3-5. As illustrated in these figures, the angularity of the material moving away from each 'of the discharge ports 26 of the supply conduits 24 corresponds substantially to the angle of repose of the particular granular material involved under the flow condition wherein the withdrawal of material from the base of the collector manifold 28 is less than the total free-fall delivery rate of material through all of the supply conduits and central outlet 58. Accordingly, as the material is withdrawn downwardly and substantially centrally from the base of the collector manifold 28 through the discharge outlet 34 in the base thereof, a controlled quantity of material will flow downwardly and out through each of the discharge ports 26 as well as the central outlet 58, dynamically maintaining the angles of repose of the material as diagrammatically illustrated in FIGURES 4 and 5. The material supplied from each supply conduit can be represented by a pie-shaped segment indicated at 62 in FIGURE 3 which, as illustrated in FIGURE 5, is convex in shape corresponding to the natural angle of repose of the material. Each such pieshaped segment 62 contributes an equal amount to the admixed material withdrawn from the base of the collector manifold, providing an accurate composite of the entire contents in the tank 10.

During the initial charging of material into the storage tank 10, a portion thereof will pass downwardly into the collector manifold until the level therein rises to a point wherein the discharge outlets of the supply conduits are blocked, after which no further withdrawal of material will occur until the gate valve assembly 38 is actuated. The material initially deposited in the collector manifold 28 will not constitute an accurate sampling of the materials unless extremely rapid loading of the tank is achieved. Accordingly, the initial withdrawal of material from the collector manifold may suitably be recirculated to the interior of the tank or discarded, as desired.

While specific embodiments of the present invention have been illustrated and described in detail above, it will be appreciated that this invention is subject to modifica tion, variation and change without departing from the proper scope or spirit of the subjoined claims.

What is claimed is:

1. A blending apparatus for uniformly mixing solid granulated materials comprising a tank, a receiver disposed below said tank, a plurality of conduits extending upwardly of said tank and formed with a series of apertures at vertically spaced intervals therealong disposed in communication with the interior of said tank for uniformly sampling and withdrawing material from said tank, said conduits and apertures being so spaced that substantially geometrically similar sections of the materials in the hopper are disposed in communication with the uppermost covered apertures of each of the conduits regardless of the level of material in the hopper, said conduits having outlet ends for discharging the material into said receiver, and means for maintaining a substantially equal flow rate of material downwardly through said conduits and in an amount such that the materials passing downwardly through said conduits is less than the free-fall rate of the materials and such that only the uppermost of the aper tures in each said conduit disposed below the level of the materials in said tank is operative to withdraw materials from said tank.

2. The blending apparatus as defined in claim 1 wherein said means comprises said outlet ends of said conduits disposed in communication with the interior of said receiver and arranged with the discharge openings thereof in symmetrical relationship and at substantially the same level, and outlet means in said receiver for withdrawing the material from said receiver at a controlled rate, said level of the material within said receiver controlled by said outlet means to remain in at least partial blocking relationship relative to said discharge openings of said conduits.

References Cited by the Examiner UNITED STATES PATENTS 407,907 7/1889- Sprague 259180 455,082 6/1891 Wilson 259-180 2,805,802 9/1957 Strong 259180 3,106,385 10/1963 Arthur et al. 259l 3,138,369 6/1964 Bennett et al 259- 3,158,362 11/1964 Seifarth 259- 3,216,629 11/1965 Goins 259180 X FOREIGN PATENTS 29,575 10/ 1931 Australia.

WILLIAM I. PRICE, Primary Examiner. 

1. A BLENDING APPARATUS FOR UNIFORMLY MIXING SOLID GRANULATED MATERIALS COMPRISING A TANK, A RECEIVER DISPOSED BELOW SAID TANK, A PLURALITY OF CONDUITS EXTENDING UPWARDLY OF SAID TANK AND FORMED WITH A SERIES OF APERTURES AT VERTICALLY SPACED INTERVALS THEREALONG DISPOSED IN COMMUNICATION WITH THE INTERIOR OF SAID TANK FOR UNIFORMLY SAMPLING AND WITHDRAWING MATERIAL FROM SAID TANK, SAID CONDUITS AND APERTURES BEING SO SPACED THAT SUBSTANTIALLY GEOMETRICALLY SIMILAR SECTIONS OF THE MATERIALS IN THE HOPPER ARE DISPOSED IN COMMUNICATION WITH THE UPPERMOST COVERED APERTURES OF EACH OF THE CONDUITS REGARDLESS OF THE LEVEL OF MATERIAL IN THE HOPPER, SAID CONDUITS HAVING OUTLET ENDS FOR DISCHARGING THE MATERIAL INTO SAID RECEIVER, AND MEANS FOR MAINTAINING A SUBSTANTIALLY EQUAL FLOW RATE OF MATERIAL DOWNWARDLY THROUGH SAID CONDUITS AND IN AN AMOUNT SUCH THAT THE MATERIALS PASSING DOWNWARDLY THROUGH SAID CONDUITS IS LESS THAN THE FREE-FALL RATE OF THE MATERIALS AND SUCH THAT ONLY THE UPPERMOST OF THE APERTURES IN EACH SAID CONDUIT DISPOSED BELOW THE LEVEL OF THE MATERIALS IN SAID TANK IS OPERATIVE TO WITHDRAW MATERIALS FROM SAID TANK. 